1. Field of the Invention
The present invention relates to electroconductive magnetic carrier particles, a developer comprising the carrier particles and an electrophotographic image formation method using such a developer for use with a printer, facsimile apparatus and digital copying apparatus.
2. Discussion of Background
According to the electrophotographic image formation method based on the Carlson process, which is now widely employed, image formation is basically carried out in such a manner that the surface of a photoconductor is uniformly charged to a predetermined polarity and the photoconductor thus charged is selectively exposed to the original light images to form latent electrostatic images on the photoconductor. Then, the latent electrostatic images are developed with a developer, so that visible toner images can be obtained on the photoconductor. The visible toner images are then transferred to a sheet of an image-receiving medium and fixed thereon.
On the other hand, many proposals on the image formation method not using the Carlson process, but using the rear side exposure system have been reported, for example, in The Journal of the Institute of Image Electronics Engineers of Japan vol. 16, (5), 306 (1987); and Japanese Laid-Open Patent Applications 61-149968, 63-10071 and 63-214781, by which rear side exposure system the image formation apparatus can be made compact and the image formation process can be made simple.
In the rear side exposure system, the surface of the photoconductor is provided with a developer to form a developer resident portion, through which the photoconductor is subjected to a cleaning operation, and the photoconductor is uniformly charged. The light images are applied to the photoconductor from the rear side thereof and the latent images formed on the surface of the photoconductor are simultaneously developed into toner images with the developer.
However, there are too many difficult problems in the rear side exposure system to put it into practice. More specifically, the requirements for each function in the rear side exposure system are made extremely severe because it is necessary to inject the electric charge sufficient for the development into the photoconductor through the developer accumulated in the developer resident portion and to form sharp and stable toner images on the photoconductor by development at a relatively small developer resident portion.
In addition, it is necessary to impart the electroconductivity to the developer since the electric charge is injected into the photoconductor through the developer. Therefore, when a developer to be employed is a one-component type developer, an electroconductive magnetic toner is essentially required. The toner image thus formed on the photoconductor cannot be transferred to a sheet of plain paper by the electrostatic image transfer method such as corona transfer or bias roller transfer. As a result, only a sheet of paper with high resistivity can be used in this system.
The method of forming a multi-color recording image on a sheet of plain paper by the rear side exposure system is disclosed in Japanese Patent Publication 60-59592. In this method, however, since a photoconductor is prepared by overlaying an insulating layer on a photoconductive layer, the photoconductor cannot stand the repetition of formation of multi-color images thereon. To solve this problem, it is proposed that the residual latent image formed on the photoconductor be erased by application thereto of a transfer electrical field. This proposal is still insufficient in practice for obtaining clear images over an extended period of time.
As in the Journal of the Institute of Electrophotography Engineers of Japan vol. 27, No. 3, p.442 (1988) and Japanese Laid-Open Patent Application 61-46961, the image formation can be achieved by the rear side exposure and the simultaneous development system, with the application of a charging bias and a development bias to a photoconductor, having counter polarities, using a two-component type developer comprising iron carrier particles with a resistivity of 10.sup.4 to 10.sup.8 .OMEGA..multidot.cm and magnetic toner particles with insulating properties.
However, when the above-mentioned image formation method is applied to the practically-used copying apparatus, it is difficult to control the image formation system for obtaining a clear image over an extended period of time, and in addition, the structure of the apparatus necessarily becomes complicated.
Furthermore, the image formation methods by use of a magnetic carrier prepared by dispersing a magnetic material in a binder resin are disclosed. For example, a developer comprising the above-mentioned magnetic carrier and an electrically insulating non-magnetic toner is employed in Japanese Laid-Open Patent Applications 53-33152 and 55-41450; and a developer comprising the above-mentioned magnetic carrier and an electrically insulating magnetic toner is employed in Japanese Laid-Open Patent Applications 53-33152, 53-33633 and 53-35546. In these disclosures, the carrier component in a developer has insulating properties and the development is carried out by the conventional Carlson process.